Resin mats, especially sheet molding compounds (SMC) have long been used in the art. Thus, the manufacture of SMC and the further processing to components have also been known for a rather long time. In the customary SMC manufacturing process a layer of a resin is applied on a foil. Then, in the next step chopped fibers of reinforcement materials—usually glass fibers—are applied onto this resin layer. The chopped fibers are produced in-line in that a rather large number of so-called glass-fiber rovings are supplied and are cut with the aid of choppers. A further resin layer and a further foil complete the SMC manufacturing process.
After the manufacture of the SMC a ripening of a few days is necessary. During this time chemical reactions take place that alter the viscosity of the resin and aid a further impregnation of the fiber layer. Then, a partial dissolution of the fiber bundle structure takes place during the following pressing operation on account of the acting shearing forces.
Preferred SMC's have a high degree of flowability that makes it possible, e.g., that the press form only has to be covered to 30 to 50%. In addition, there is the achievable surface quality, which is possible up to class A levels. Resin mats that comprise a fabric or a fiber nonwoven of continuous fibers frequently have a reduced flowability. Accordingly, such resin mats must be introduced into the press form with great precision. Furthermore, the surface quality of the form parts produced can be disadvantageously influenced by such reinforcing mats.
The presented manufacture of SMCs with chopped fibers is as a rule a relatively complex and slow process. As described, in-line cut glass fibers are generally used for it that are produced on site from so-called rovings. As a consequence, a large number of spools, supply equipment and other devices are required that require a significant amount of work and can readily result in technical problems. As a result, problems of quality and yield immediately result, e.g., by a standstill of the machines, that frequently cause significant expenses. Furthermore, the cutting of the fibers limits the speed of the manufacture of SMC's.
WO99/55521 attempts to generate a glass-fiber layer with a wet deposition method in an in-line process. This method is expensive and requires extensive additional measures on account of the necessary drying. Furthermore, significant properties of the glass-fiber nonwovens are not described.
WO001/19599 describes a method in which a layer of unidirectional reinforcement fibers are inserted in the SMC process that are additionally covered with chopped fibers. This method is particularly suited for applications that take into consideration the special requirements for strength in a preferred direction. Moreover, significant properties of the glass-fiber nonwovens are not described.
Finally, WO 2005/254559 describes a method for the manufacture of needled glass-fiber mats that can then also be used for SMC-like components. However, the needling of a mat constitutes an additional and expensive process step. Furthermore, substantial properties of the glass-fiber nonwovens are not described. Thus, a needling potentially results in particular in a very permanent solidification of the nonwovens. Therefore, the flowability of the resin mats manufactured from these glass-fiber mats is relatively low.
Document US2005/0082721 describes mats having glass fibers and plastic fibers. However, this document contains no suggestions concerning reaction resins. Moreover, significant properties of the mats are not described.
The European patent application (EP JM05002) discloses glass-fiber nonwovens and their use as resin mats in SMCs, the glass-fiber nonwovens having an inhomogeneity of at the most 10%, a weight per unit area of at least 600 g/m2 and a strength of at least 10 N. The required strength of at least 10 N means that the glass-fiber nonwoven must be strengthened with a binder or by other strengthening methods (e.g., needling). It is possible with the aid of glass-fiber nonwovens designed in this manner to manufacture them in advance and introduce them into the SMC process. In this manner the in-line cutting of the rovings can be eliminated.
On the whole, it should be determined that the current resin mats frequently satisfy the given purpose. However, the manufacture of these resin mats is expensive, complex and subject to errors. Moreover, the method can not be carried out in a flexible manner.
Therefore, there was the task of indicating a method with which resin mats can be economically and reliably manufactured. The resin mats obtained in this manner should be able to be processed to form parts with a high surface quality.
Furthermore, the resin mats should be able to be processed in a simple manner and the resin mats should exhibit a high flowability.